I mention this frequently so I better explain what it is...

Mastuyani often just referred to as "pitch" is mixture of pitch and clay that is used to secure a metal part like a sword guard. The use of tagane requires both hands. One to hold the hammer and one to hold the chisel or punch. So you need something that can firmly hold your piece without denting or scratching the surface. 
Matsuyani is hard like black concrete. It melts easily and this how you attach your work. Using a torch or hot metal you can make the pitch soft and sticky. I just drop my work piece into the hot pitch and let it cool. It sticks to metal like glue. When it has cooled off it does not stick to your hands but still holds the piece firmly.
The pitch does not clamp the piece in any way so it does not leave the slightest mar in the surface. A little heat and the piece can be pulled free. Any remaining Mastuyani can cleaned off with organic solvent with no ill effect to the metal.
Because of this, one side of a tsuba can be worked from rough carving all the way through polish. Then it can be flipped. The polished side being completely protected by the Matsuyani. The second side can then be work without risk to first side.

Matsuyani is made from pine rosin, powdered clay, oil, and powdered charcoal. Pine rosin is inexpensive and readily available, but I still prefer to harvest natural flows of resin from trees. I render this to make home made pine rosin. I get a greater sense of satisfaction out of harvesting and processing my own ingredients.

Boiling off the turpines out of pine sap to make pine rosin.

Melting the rosin chunks to mix in the other ingredients. When its ready I take a sample out and let it cool to test the properties.